What is adjusted body weight?

Adjusted body weight is a dosing weight for obese patients that sits between ideal and actual weight. It is calculated as ideal body weight plus 0.4 times the gap between actual and ideal weight, recognising that some drug distributes into excess adipose tissue but not all of it.

Why use 0.4 as the correction factor?

The 0.4 factor reflects that hydrophilic drugs partition partially into fat. About 40 percent of the excess weight over ideal contributes to the drug's volume of distribution. Some references use slightly different factors for specific drugs, but 0.4 is the common default.

When should adjusted body weight be used?

It is generally applied when actual weight is more than about 20 to 30 percent above ideal weight. Typical uses include aminoglycosides, unfractionated heparin, and low-molecular-weight heparin. For patients near or below ideal weight the adjustment is not needed.

What happens if actual weight is below ideal?

Then there is no excess weight to adjust for, so the adjustment is not applied and ideal body weight (or actual weight, if even lower) is used. This tool detects that case and reports that ideal weight should be the dosing basis.

Which ideal-weight formula does this use?

It uses the Devine formula to compute ideal body weight: 50 kg for men or 45.5 kg for women, plus 2.3 kg per inch of height over 60 inches. The adjusted weight is then layered on top of that figure.

Adjusted Body Weight Calculator

Adjusted body weight bridges the gap between ideal and actual weight when dosing drugs in obese patients. Ideal body weight alone can under-dose, because part of a hydrophilic drug does distribute into excess tissue, while actual weight can over-dose. The adjustment captures a defensible middle figure.

How it works

The calculation is built in two steps. First the Devine ideal body weight, then the adjustment:

IBW (male)   = 50.0 + 2.3 x (height_in - 60)
IBW (female) = 45.5 + 2.3 x (height_in - 60)
ABW          = IBW + 0.4 x (actual weight - IBW)

The 0.4 correction factor assigns 40 percent of the weight above ideal to the drug’s volume of distribution. When actual weight is at or below ideal weight, there is nothing to add, so ideal weight is used directly.

Example and notes

A 170 cm man has a Devine IBW near 65.9 kg. At an actual weight of 110 kg, the adjusted body weight is 65.9 + 0.4 x (110 - 65.9), roughly 83.5 kg. That figure, not the full 110 kg, would be used for an aminoglycoside dose. The adjustment is most relevant once a patient exceeds ideal weight by 20 to 30 percent; below that threshold the difference is small.